1. Field of the Invention
The present invention relates to a semiconductor device, such as a solid-state image sensing device which is used in an image input device such as a camera and has an interlayer lens at a position corresponding to a photoelectric conversion portion, and a method for fabricating the same.
2. Description of the Related Art
Such a solid-state image sensing device includes, for example, a charge coupled device image sensor (hereinafter, simply referred to as CCD), a complementary metal oxide semiconductor (CMOS) image sensor, and the like. These types of solid-state image sensing devices are used in various applications, for example, a digital camera, a video camera, a mobile telephone with an integrated camera, a scanner, a facsimile, and the like. As a device using such a solid-state image sensing device is becoming more widely used, there is a stronger demand not only in improving the function and performance, for example, increasing the number of pixels, enhancing light receiving sensitivity, but also in miniaturizing the device and reducing the cost.
As a solid-state image sensing device is becoming smaller and the number of pixels is increasing while the cost is kept low, the size of each of the pixels is becoming smaller and smaller. As the pixel size decreases, the light receiving sensitivity, one of the basic properties of the solid-state image sensing device, deteriorates. Thus, it is difficult to acquire a picture of a vivid image with a low illuminance. Therefore, how to improve the light receiving sensitivity per unit pixel is an important challenge to be tackled.
As a method for improving the light receiving sensitivity of a solid-state image sensing device, Japanese Publication for Opposition No. 2945440 discloses a method for forming a microlens on a color filter from an organic high polymer material. Further, Japanese Laid-Open Publication No. 11-40787 discloses a method for forming a so-called interlayer lens inside a laminate structure below a color filter and between a light-receiving portion and the color filter as shown in FIG. 4.
As shown in FIG. 4, a conventional solid-state image sensing device includes a photoelectric conversion portion (light receiving portion) 22, a read out gate portion 23, a CCD transfer channel 24, and a channel stopper 25, which together form a pixel, on a surface of a semiconductor substrate 21.
A transfer electrode 27 is formed on the CCD transfer channel 24 via an insulation film 26. A light shielding film 29 is formed on the transfer electrode 27 via an interlayer insulation film 28. A first flattening film 30 formed of boro-phospho-silicate glass (BPSG) and the like, and an interlayer lens 31 formed of a silicon nitride film and the like are formed on the light shielding film 29. Then, a surface is flattened by a second flattening film 32. A color filter 33 which is a combination of red green and blue (R, G, and B) is formed on the second flattening film 32. A microlens 35 is formed on the color filter 33 via a protection film 34 so as to be located above the photoelectric conversion portion 22.
As another method for improving the light receiving sensitivity of a solid-state image sensing device, Japanese Laid-Open Publication No. 2001-60678 discloses a microlens having a variable refractive index which utilizes a characteristic that the refractive index varies depending on a voltage applied (pockelse effect).
When a solid-state image sensing device is integrated in a video camera, the f-number of a lens provided at the video camera side varies to have the appropriate exposure depending on the lighting conditions. Thus, light incident on the solid-state image sensing device through the lens of the video camera has an angle varied by a diaphragm of the lens of the video camera. The light incident on the solid-state image sensing device is not only collimated light but also diagonal light. Therefore, electrodes are provided on and under the microlens or an interlayer microlens so that the incident light is always received at a photoelectric conversion portion in accordance with the diaphragm of the lens of the video camera. The refractive index of the microlens or interlayer microlens is made to be flexibly changed by applying a voltage to the microlens or interlayer microlens. In this example, the material used for the microlens or interlayer microlens is a refractive-index variable layer formed of electrooptic ceramics (for example, PLZT, LiNbO3). PLZT is a piezoelectric material which is lead titanate-lead zirconate solid solution (PbTiO3-PbZrO3) in which a part of Pb is substituted with La.
Hereinafter, a generally known method for forming an interlayer lens 31 will be described.
First, as shown in FIG. 5A, predetermined impurity ions are injected into a semiconductor substrate 21 to form a photoelectric conversion portion 22, a read out gate portion 23, a CCD transfer channel 24, and a channel stopper 25.
Then, an insulation film 26 is formed on a surface of the semiconductor substrate 21. A transfer electrode 27 having a predetermined pattern is formed on the insulation film 26 to a film thickness of 300 nm. A light shielding film 29 is formed on the transfer electrode 27 via an interlayer insulation film 28 to a film thickness of 200 nm. The light shielding film 29 covers the transfer electrode 27 and has an opening above the photoelectric conversion portion 22.
Next, as shown in FIG. 5B, a BPSG film having a predetermined phosphorus concentration and boron concentration is deposited by a normal pressure chemical vapor deposition (CVD) method to a film thickness of about 600 nm on the light shielding film 29. Then, under a reflow at a high temperature of 900° C. or higher, a first flattening film 30 is formed.
Then, as shown in FIG. 5C, a silicon nitride film 36 is formed on the first flattening film 30 by a plasma CVD method to a film thickness of about 1200 nm. For forming a microlens or an interlayer microlens from the above-mentioned refractive-index variable material, a sputtering is performed with (Pb, La) (Zr, Ti) O3 as a target and under the conditions of gas sources and flow rates, Ar: 100 sccm and O3: 10 sccm, to form a PLZT film. An example in which a silicon nitride film is formed will be described below.
As shown in FIG. 5D, a positive resist is applied to the silicon nitride film 36. A patterning is performed to obtain a desirable interlayer lens 31. Then, under a reflow around 160° C., for example, a resist pattern 37 having a lens shape is produced.
Then, as shown in FIG. 5E, the lens shape of the resist pattern 37 is transferred to the silicon nitride film 36 by performing a dry etching with strong anisotropy to form the interlayer lens 31.
In order to improve the condensing rate of the interlayer lens 31, a second flattening film 32 is formed to flatten a surface as shown in FIG. 4. The second flattening film 32 is made of a material having a low refractive index and covers the interlayer lens 31. Then, a color filter 33, a protection film 34 and a microlens 35 are formed to produce a conventional CCD-type solid-state image sensing device.
The above-described methods for forming an interlayer lens have the following problems 1 to 3.
1. The silicon nitride film 36 formed as the interlayer lens 31 is formed by using a normal pressure CVD device, or a plasma CVD device, in general. When the silicon nitride film 36 is formed to have a film thickness of 1000 nm or higher, the film experiences plastic deformation due to film stress. This may cause a crack or removal of the film. Further, a film formation temperature is 200° C. or higher. The uniformity of the film thickness within a plane of a wafer is not high. Thus, there is a bad influence on underlying elements due to a high temperature or a film stress, or the film thickness of the inter layer lens becomes uneven. This causes deterioration of picture quality of the solid-state image sensing device.
2. The refractive index of the interlayer lens 31 formed of the silicon nitride film 36 is about 2.0 at most. For example, when an oxygen content is increased when the film is formed, the compound becomes SiON and the refractive index is reduced to about 1.5. When the size of the video camera is to be reduced, it is necessary to improve the condensing rate (a percentage of the amount of light to enter the photoelectric conversion section from the total incident light) and also to reduce the distance from a surface of the image sensing device to the photoelectric conversion section in order to support a short pupil position lens. In order to achieve this, it becomes necessary to make the microlens, the color filter, and also the interlayer lens thinner. For improving the condensing rate while making the interlayer lens thinner, the refractive index of the interlayer lens has to be increased. If a silicon nitride film is formed with a conventional CVD method, it is extremely difficult to form a transparent film having a refractive index of 2.0 or higher with high uniformity.
3. When a refractive-index variable microlens or an interlayer lens is formed by forming a refractive-index variable material layer made of electrooptic ceramics such as PLZT, LiNbO3 or the like by a sputtering method, it is required to form transparent electrodes above and below the lens and process wiring for voltage application. This causes the problems that the process steps become complicated and the production cost increases. Further, when such a refractive-variable microlens is incorporated into a solid-state image sensing device, the refractive index of the lens can be varied within the range of about 2.2 to 2.6 under the conditions that the transparency is maintained upon voltage application and no mechanical distortion is produced. Therefore, in terms of both the performance and the cost, it is more advantageous to design the thickness, the shape, the refractive index and the like of the microlens or interlayer microlens for every application of the video camera, such that diagonal light can efficiently enter the photoelectric conversion portion when the diaphragm of the lens is opened, so as to conform to design of the video camera to which the solid-state image sensing device will be mounted and produce such a microlens or interlayer microlens, than it is to make the refractive index variable depending on the diaphragm of the camera lens.